1. Field of the Invention
The present invention relates to an image sensor, and more particularly, a complementary metal-oxide semiconductor (CMOS) image sensor and a method for fabricating the same in which condensing efficiency of light is maximized to improve characteristics of the image sensor.
2. Discussion of the Related Art
An image sensor is a semiconductor device that converts optical images to electrical signals. The image sensor is classified into a charge coupled device (CCD) image sensor and a CMOS image sensor.
The CMOS image sensor includes a photodiode area sensing light and a CMOS logic circuit area processing the sensed light to generate electrical signals. If light-receiving amount of the photodiode is great, the image sensor has excellent photosensitivity characteristics.
To enhance photosensitivity, it is necessary to increase a fill factor of an area occupied by the photodiode among the whole area of the image sensor. Alternatively, it is necessary to change a path of incident light to an area other than the photodiode so as to concentrate light to the photodiode.
To concentrate light to the photodiode, a microlens is generally used. A convex microlens made of material having good light transmittance is formed on the photodiode to refract a path of incident light, thereby irradiating more light to the photodiode.
Light parallel to a light axis of the microlens is refracted by the microlens so that a focal point is formed on a certain position of the light axis.
Hereinafter, a related art CMOS image sensor will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a related art CMOS image sensor.
As shown in FIG. 1, the related art CMOS image sensor includes at least one or more photodiodes 11 formed in a semiconductor substrate (not shown) to generate charges in response to incident light, an inter-dielectric layer 12 formed on an entire surface of the semiconductor substrate including the photodiodes 11, a passivation layer 13 formed on the inter-dielectric layer 12, R/G/B color filter layers 14 formed on the passivation layer 13 to respectively pass through light of specific wavelengths, a planarization layer 15 formed on the color filter layers 14, and a convex microlens 16 having a certain curvature formed on the planarization layer 15 to pass through the color filter layers 14, thereby concentrating light on the photodiodes 11.
Although not shown, the CMOS image sensor further includes a light-shielding layer in the inter-dielectric layer 12 to prevent light from entering a portion other than the photodiodes 11.
Instead of the photodiode, a photogate type device may be used as a device for sensing light.
The curvature and the height (‘A’ of FIG. 1) of the microlens 16 are determined considering various factors such as a focal point of concentrated light. A resin such as polymer is generally used as the microlens 16. The microlens 16 is formed by deposition, patterning and reflow processes.
The optimized size and thickness and the curvature radius of the microlens 16 should be determined by size, position and shape of a unit pixel, a thickness of the photodiode, and height, position and size of the light-shielding layer.
A photoresist is generally used as the microlens 16 whose curvature and height are determined considering various factors such as a focal point of concentrated light. The photoresist is coated and then patterned by exposing and developing processes to form a photoresist pattern. The photoresist pattern may be formed by a reflow process.
Meanwhile, a pattern profile is varied depending on focus of the photoresist.
For example, process conditions are varied depending on a condition of a sub-layer. Therefore, a profile of the microlens is varied.
In the process for fabricating the aforementioned related art CMOS image sensor, the microlens 16 formed to improve concentration efficiency of light serves as a main factor that determines characteristics of the image sensor.
The microlens 16 serves to condense more light to the photodiodes 11 through the respective color filter layers 14 depending on wavelengths of light when the light is irradiated.
The incident light to the image sensor is concentrated by the microlens 16 and filtered by the color filter layers 14. The light then enters the photodiodes 11 corresponding to the color filter layers 14.
The light-shielding layer serves to prevent the incident light from getting out of its path.
When the related art CMOS image sensor is fabricated, since the condition of the photoresist pattern is unstable, concentration efficiency of the light is deteriorated. This could deteriorate performance of the CMOS image sensor.